The invention relates to vision systems for accurately sensing the position and condition of leads used on integrated circuits prior to placement of the integrated circuit on a circuit board by a surface mount pick and place machine. More particularly, the invention relates to a non-contact laser-based sensor system which can, with the highest degree of resolution, determine lateral orientation and coplanarity of leads for all such integrated circuit components, even those having an ultra-fine pitch.
As electronic devices get smaller and smaller, and yet more complex, electronic components which are connected together to achieve the electronic functions of such devices are also decreasing in size. Additionally, many functions are being incorporated in single unitary monolithic integrated circuits. These integrated circuit components, which are also called semiconductor packages or chips, have a number of leads or elements providing electrical connections. As a consequence of their small size, integrated circuits are created in a configuration referred to as quad flat packs (QFPs) or quad packs with closely spaced leads emanating from each side of the quad pack. QFPs are particularly configured for surface mount placement with special leads which are referred to as gull wings. The QFPs are precisely placed by a pick and place machine upon the surface of a circuit board with the gull wing leads making proper contact with the circuit connections or pads which are subscribed on the circuit board or work piece. Similarly, more and more manufacturers are creating integrated circuits with even more delicate leads structured on tape, which is used in a process called tape automated bonding (TAB).
The separation between the centers of any pair of adjacent leads on such components is referred to as the pitch. Currently, a commonly manufactured component separation is 25 mil pitch, the center of each lead spaced at 25 thousandths of an inch intervals. However, advances in component manufacturing technology have produced integrated circuits having 15 and 10 mil pitches and TAB components have been created having several hundred leads spaced with a 4 mil pitch.
For automated manufacturing of electronic devices using these microelectronic components, the highest degree of accuracy in positioning and placing such components is required. To perform this delicate task, precision surface mount component placement machines have been developed.
There are two types of component placement machines in common use today. One of which is a cartesian system where one or more vacuum quills are used to travel to a bin, pick up a component, properly orient the component and carry it to a circuit board or other work piece to precisely place the component in its proper location. The proper location is the one where the leads make proper contact with the circuit connections which are subscribed on the circuit board or work piece. Another type of placement system in use is a carousel or turret placement system where components are picked up from the bin and stepped through stations located around the perimeter of a circular component carrying mechanism for placement on the circuit board. The components are not aligned in the component bins. Typically, gull wing type components may be out of position by plus or minus 50 mils and plus or minus 5 degrees angular orientation. Therefore, the orientation of components from bins must be determined prior to placement. The present invention is useful with both types of systems which must accurately place components with the highest degree of speed and accuracy.
Surface mount circuit boards have a number of small individual pads. Each lead from each electrical component must be placed precisely on one circuit board pad, to ensure proper electrical contact, thus requiring correct angular orientation and lateral positioning of the component. The dimensions of components to be placed normally vary between 0.02 inch and 2.0 inches.
In a surface mount component placement machine, a transport arm picks up the component from a component bin utilizing a vacuum quill as the primary instrument which gently picks up the component to be placed and transports it between the component bins and the circuit board. The transport arm moves the component from the bin to the circuit board located on a work table. Most component placement machines have a built in position encoder which electromechanically determines the position of the placement head. The vacuum quill is attached to the placement head and is moved from the component bin to the circuit board by the placement head. Therefore, the component placement machine knows the position of the quill based upon the encoder reading. The encoders generally provide readings within plus or minus 4 microns. The accuracy of the encoders is a limitation of the component placement machine's ability to accurately place components on a circuit board. During transport, the angular orientation of the component and the offset of the component from the center of the quill are determined. The condition of the leads is also checked to determine whether any are bent or skewed. Any necessary corrections in placement are then calculated and the placement head is adjusted to accommodate the corrections. The vacuum quill is then precisely lowered to fit the component on the circuit board. In current component placement machines, the transport arm and quill move at approximately one meter per second. However, the speed of movement may range from zero to 8 meters per second. At this speed, it is difficult for current systems to handle the fine pitch and range of components that must be placed and to achieve the accuracy required for alignment.
For quality manufacturing, component leads must be placed with at least 80% overlap of lead onto the corresponding pad of the circuit board. A device having a 20 mil pitch generally has 10 mil wide leads. With an 80% overlap, at least 8 mils of the lead width must be on the pad with no more than 2 mils of the lead width off the pad. In general, sensing systems used to align parts for placement must have five to ten times better resolution than the accuracy required. Therefore, 0.2 to 0.4 mil image resolution is required to achieve the maximum placement error of 2 mils specified for quality manufacturing methods.
The leads on integrated circuits are manufactured in a variety of ways, such as stamping and etching. The shape of the leads created from different manufacturing methods varies. One method of forming leads is to apply photoresist to the surface of the metal and expose it to light. When exposed to light, the photoresist chemically etches through the metal to create the leads. The etch rate may be such that the lead is not formed with substantially vertical walls. Rather, the walls take on a scalloped shape, with the lead cross section assuming a substantially trapezoidal shape. For instance, if leads are etched from the top down, their sides are jagged and their lower or base surface is larger than the upper surface. These irregularities in lead shape require lead sensing equipment to be specially configured in order to handle the different shapes.
Quad flat packs (QFPs) can be manufactured with bumpers that extend outward beyond each corner of the component and beyond the leads of the component. The bumpers assist in protecting leads from being bent out of position. Bumpered QFPs present new problems for lead sensing equipment. The bumpers, in providing physical protection to keep the leads from being bent, block conventional vision sensing systems. The bumpers may extend as far as 8 microns beyond the leads.
Conventional vision systems used in conjunction with component placement machines for lead determination use a solid state television camera having a resolution of 512.times.512 picture elements or pixels. A two inch part and a corresponding two inch field of view with 512 elements produces a basic resolution of 4 mils or 4 thousandths of an inch. This is not a sufficient resolution and, in fact, as pointed out above, it is necessary to achieve a resolution which is at least an order of magnitude greater. One solution that has been suggested is the use of multiple lenses. Using different lenses to achieve the requisite magnification provides multiple fields of view. However, changes in the lens system consumes time, slowing the placement process and, thus, trades speed for accuracy.
Another solution that has been proposed is the use of super-resolution. With backlighting, a shadow is cast on the solid state pixel elements of a detector array and, by applying gray scale image processing algorithms to the intensity information of shadow edges cast upon the detector array, a super-resolution can be achieved which is approximately four times the resolution that can be achieved with binary processing.
Yet another solution proposed is a two-dimensional detector array having as many as 2,000.times.2,000 elements. Such a product is manufactured by Eastman Kodak of Rochester, N.Y. However, this solution is prohibitively expensive.
Finally, the use of linear arrays having 1,000-2,000 detector elements is also possible. Using such a system, the motion of the part is synchronized with continuously read successive frames from the array to build up to a two-dimensional image. With super-resolution, an accuracy of resolution is achieved equivalent to that needed for a 20 mil pitch device.
What is needed is an economical system which can accurately locate leads having 15, 10, and even 4 mil pitch. These systems must have an accuracy of resolution in the range of at least 0.02 mils. What is needed is a system which can accurately locate leads of various shapes and sizes. What is needed is a sensing system where the light is not obscured from reaching the leads by bumpers which provide physical protection to keep the leads from being bent. The present invention is addressed to fulfill these needs.